1. Field of the Invention
The present invention relates to an apparatus and method for detecting a voltage of an assembled battery, and particularly to an apparatus and method for detecting voltages of single batteries in an assembled battery, the apparatus and method being capable of detecting a charge state of a number of single batteries.
2. Description of the Related Art
In an electric vehicle or in a hybrid electric vehicle (HEV), a motor is used as a driving power source. As an electric power source of such a motor, there are used an assembled battery made by serially connecting single batteries such as lithium secondary batteries, fuel batteries or the like.
In particular, the lithium secondary batteries are vulnerable to overcharging or over-discharging. If the batteries are not used at a voltage within a predetermined use range, there is a danger that they cannot be used because a material decomposes, a capacity is significantly lowered, and abnormal heating occurs.
Therefore, in the case where the lithium secondary batteries are used to form an assembled battery, it is required to sufficiently restrict and precisely detect dispersion in voltages of single batteries so as not to occur overcharging or over-discharging in single batteries that configure the assembled battery.
In addition, in the case of fuel batteries which do not require charging as well, it is required to precisely detect output voltages of all single batteries, prevent pole transfer, and prevent shutdown due to an excessive load in order to continuously and efficiently maintain electric power generation. In addition, it is also required to precisely detect output voltages of the fuel batteries in order to detect and control fuel jamming for continuously and efficiently maintaining electric power generation.
In the case of conventional lead batteries using a water soluble electrolyte or an assembled battery composed of single batteries such as nickel batteries, these batteries have properties that dispersion between single batteries is eliminated (uniformly charged) to a certain extent. Thus, over-discharging or overcharging of single batteries have been successfully prevented by monitoring a voltage across a certain single battery and controlling charging and discharging so that the monitored voltage is maintained in a predetermined voltage range.
However, in an assembled battery using lithium batteries as single batteries, the assembled battery being composed by using an organic electrolyte, uniform charging is not carried out under such a control. Thus, it is known that dispersion between single batteries expands, and overcharging or over-discharging of single batteries proceeds, causing performance deterioration such that an unusable state occurs.
Therefore, in a conventional assembled battery using lithium batteries as single batteries, there is provided a charge state detecting apparatus for detecting whether or not a charge state of each of the single batteries as shown in FIG. 8 is controlled to be charged or discharged within a predetermined set voltage range, i.e., abnormal overcharging or over-discharging that exceeds the predetermined set voltage range (for example, refer to FIG. 1 and page 1 of Jpn. Pat. Appln. KOKAI Publication No. 2003-32907).
This charge state detecting apparatus is comprised of; a voltage detecting circuit 15a for detecting overcharging or over-discharging of single batteries in an assembled battery 11 made of a number of single batteries Ba1 to Ban; and an overcharging or over-discharging judging circuit 15b equipped with a logic circuit for judging overcharging or over-discharging from an output of the detecting circuit.
In addition, in the voltage detecting circuit 15a, there are provided, by number of single batteries, resistors RUa and RUb of a voltage dividing circuit for supplying a voltage to judge overcharging by single batteries; resistors RLa and RLb of a voltage dividing circuit for supplying a voltage to judge over-discharging; a constant voltage circuit Er for setting a comparator voltage to judge overcharging or over-discharging; a differential amplifier COU for detecting overcharging from the voltages supplied from these voltage dividing circuits and the comparator voltage set by the constant voltage circuit Er; and a differential amplifier COL for detecting over-discharging.
Further, each one of the differential amplifier COU and the differential amplifier COL is placed at an electric potential with respect to the ground potential, which differs from single battery to battery. Thus, offset voltages of the differential amplifiers COU and COL themselves and an effect of a voltage error of the respective constant voltage circuits Er for setting a reference voltage to be compared cannot be ignored, and there is a problem that the adjustment for controlling the voltage error becomes complicated. Thus, a configuration of this charge state detecting apparatus becomes complicated and expensive, and the detecting apparatus has been unsuitable for voltage detection of an assembled battery made of a number of single batteries.
In contrast with such a charge state detector section for an assembled battery, there is provided a voltage detecting apparatus for an assembled battery having a configuration, called a flying capacitor system, such that reference electric potentials of single batteries Ba1 to Ban, are fixed to a ground potential so as not to be affected by the different reference voltages and such that the number of differential amplifier circuits COU, differential amplifier circuits COL, and voltage setting parts is reduced for judging the above-described overcharging or over-discharging (for example, refer to FIG. 1 and page 1 in Jpn. Pat. Appln. KOKAI Publication No. 2001-201522).
This flying capacitor system is featured by reducing an effect of a parasitic capacitance of a sampling switch section 22 made of a plurality of switches for changing a connection destination of a capacitor for charging each of the single batteries Ba1 to Ban and detecting a voltage of each of the single batteries with high precision.
This voltage detecting apparatus is configured as shown in FIG. 9, and comprises a sampling switch section 22 for sampling voltages of the single batteries Ba1 to Ban of an assembled battery 11 at a capacitor C; a transfer switch section 24 for transferring each of the single battery voltages charged in the capacitor C; a voltage detecting circuit 25 for detecting a charge voltage of the capacitor C; and a controller 26 for controlling a switching timing of each switch of the sampling switch section 22 and the transfer switch section 24 so as to reduce an effect of the parasitic capacitance, and then, detecting abnormal over-discharging or overcharging from an output of one voltage detecting circuit 25.
In this system, in the case of measuring a voltage of the single battery Ban of the assembled battery 11, for example, a sampling switch S22nL and a sampling switch S22nH are first closed at the same time, and the capacitor C is sampled and held or charged with the voltage of the single battery Ban.
Next, the sampling switch S22nL and the sampling switch S22nH are opened at the same time. Then, a ground potential setting switch S24L is closed. After the lower side of the capacitor C has been transferred to the ground potential, the switch S24H is closed with a slight delay. At this time, a charge voltage of the capacitor C is detected by means of the voltage detecting circuit 25, and the switch S24L and switch S24H are opened at the same time.
Then, this operation is sequentially made with respect to all of the single batteries Ban−1 to Ba1 one by one so as to detect the presence or absence of overcharging or over-discharging with respect to each of the single batteries.
However, in the case of a charge voltage detecting apparatus shown in FIG. 9, the voltages of the single batteries Ba1 to Ban are sequentially charged and detected with respect to one capacitor C. That is, the capacitor C is charged while sampling switches S221L to S22nH are sequentially switched, and a voltage of the charged capacitor C is read out, thereby obtaining the voltages of the single batteries Ba1 to Ban. However, in such a flying capacitor system, in the case where a load current of the assembled battery fluctuates while detection of charge voltages is carried out with respect to all the single batteries, there is a problem that dispersion in individual voltages cannot be precisely obtained.
That is, in the voltage detecting circuit, it is important to precisely obtain dispersion in voltages of the single batteries Ba1 to Ban. However, a sampling time is different from another one, in the case where a load current has changed, the voltages of the single batteries Ba1 to Ban having different internal resistances change, thus making it impossible to correctly obtain a difference in relative voltages of the single batteries Ba1 to Ban.